The HQ-180 has a two-position BFO switch labeled AM (off) and CW/SSB
(on). When the BFO is "on" the beat frequency is adjustable using the
"BFO KCS" knob. When using the BFO to receive SSB it in necessary to
set the BFO KCS switch to 0 (zero) for the sideband selector switch to
operate correctly (the BFO must be tuned to the IF frequency). The
HQ-180A added a "fixed frequency" position to the BFO switch that cut
the BFO KCS switch out of the circuit and puts the BFO frequency at
the proper value for SSB reception. Hammarlund did this because
people wouldn't read the instruction book and learn how to operate the
receiver. Apparently they got complaints about the sideband selection
not working when the problem was the BFO was not adjusted properly.
The extra BFO switch position was added to make the HQ-180A idiot
proof. If you can read and understand the manual, you don't need the
extra switch position.

The accessory socket could be used to power up a 2/6 meter converter
or similar accessory. Probably of limited use for most folks.

In principle, the solid-state power supply should provide a little
better regulation. My HQ-180 is as stable as any other tube-type
receiver I own (about 35 or so), so I don't think stability of the 180
is a big issue.

I suppose if I had a choice I would pay $50 or so more for a 180A.
There seem to be many more 180 receivers out there than the 180A
versions. --Ed